Rick Stewart sweats in a park in Weehawken, N.J., with the New York City skyline in the background (Seth Wenig/AP)

There are very few positive ways to describe the month of September. It was hot, steamy, oppressive and smelly. The mud was never-ending. The mushrooms are still growing. The most generous thing I can say for the month is that it’s finally over.

I saw this ugly map today, and I thought it really hit the nail on the head. Brown colors show where overnight temperatures were warmer than normal in September. Where the swampy hues coincide with a “1,” it means the location had the warmest nighttime temperatures on record last month. In general, the lower the number, the more oppressive the nights in September were.

It takes only a cursory scan to see that September was miserable across the eastern United States — at least after the sun went down. There probably won’t be much argument if we say it was miserable during the day, too. They are connected by the two things that made last month particularly unbearably: clouds and humidity.

(Iowa Environmental Mesonet/The Washington Post)

This summer was beyond humid — it was unbearably muggy. The dew point, which is how meteorologists measure humidity, was above 70 degrees for a record-setting duration this year in Boston; Hartford, Conn.; Portland, Maine; and Providence, R.I. Some of the moisture was coming from bathlike water temperatures off the Northeast coast. Some of it was blowing north from the Gulf of Mexico.

Humidity has an interesting affect on temperature. Although moisture on your skin helps to cool your body when it’s hot out, moisture in the air prevents the temperature from dropping overnight. Water vapor is a greenhouse gas, meaning it has heat-trapping properties. When it’s very humid at night, less of the day’s heat can escape to space.

The other element that prevents heat from escaping at night is clouds. Depending on the height of the cloud, its thickness and the time of day, it can have a warming or cooling effect. Overall, at night, clouds tend to have a net warming effect. They trap heat near the surface of the Earth, preventing overnight temperatures from cooling.

These two factors — clouds and humidity — are what contributed most to September’s gross weather. I’m ready for fall now.